Frederick , Maryland -LRB- CNN -RRB- -- The Federal Aviation Administration on Friday told 149 regional airports across the country it would begin closing their air traffic control towers in April , but said it would spare another 40 towers that had been on the chopping block .

The agency said the cuts are necessary to help meet $ 637 million in forced spending cuts .

The tower closures will begin April 7 and will be phased in over four weeks , FAA Chief Operating Officer David Grizzle said in a memo obtained by CNN . Some towers were spared after the FAA considered appeals from towers , and consulted with the departments of defense and homeland security .

Among those to be closed are towers at regional airports in Frederick , Maryland , and near St. Petersburg , Florida , that were built with federal stimulus money and have been open less than a year .

Read the full list of tower closures

The decision `` will have both short-term and long-term effects , '' warned the National Air Traffic Controllers Association .

`` These towers serve other important functions -- including law enforcement activity , medical transport flights , search and rescue missions , business and commerce and supporting flight schools across America , '' the association said in a statement .

`` Ultimately , the partisan posturing in Washington that led to sequestration is the reason for -LRB- Friday 's -RRB- decision and its destructive effects on aviation . ''

The FAA had been expected to announce the closure of 189 low - or moderate-volume towers staffed by contractors . Before Friday 's announcement , it said it would consider keeping a tower open if the airport convinces the agency it is in the `` national interest '' to do so .

New air traffic control towers among those on chopping block

A news release from the FAA quoted Transportation Secretary Ray LaHood as saying , `` We heard from communities across the country about the importance of their towers and these were very tough decisions ... Unfortunately we are faced with a series of difficult choices that we have to make to reach the required cuts under sequestration . ''

By congressional mandate , the FAA must cut nearly $ 600 million from its nearly $ 48 billion budget this fiscal year . Because the majority of its 47,000 employees are air traffic controllers , it is impossible to cut its budget without affecting controllers , the agency said .

FAA Administrator Michael Huerta said in the agency 's news release that the agency `` will work with the airports and the operators to ensure the procedures are in place to maintain the high level of safety at non-towered airports . ''

Two Republican members of Congress -- Sen. John Thune of South Dakota , ranking member of the Senate Committee on Commerce , Science and Transportation ; and Rep. Bill Shuster of Pennsylvania , chairman of the House Committee on Transportation and Infrastructure -- wrote to LaHood calling for `` a detailed justification and explanation for how the FAA determined each contract tower would be closed . ''

`` It is worth noting that , the specified towers in the FAA Contract Tower Program were all operational in 2009 , when the FAA received less funding than it will under sequestration , '' the two said in their letter .

Their was no immediate response from LaHood or the FAA to the letter .

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CNN 's Deirdre Walsh also contributed to this report .

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Transportation secretary calls it `` difficult choices that we have to make ''

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FAA administrator says agency with work with affected airports to ensure safety

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Controllers ' group says closures will hurt `` overall margin of safety for our entire aviation system ''

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Among towers are two built with federal stimulus money and open less than a year